Colorectal cancer is the third most common type of cancer in the United States and the second leading cause of cancer-related mortality among men and women combined. Rectal cancers are located in the rectum deep in the pelvis, and it is estimated that there will be approximately 40,000 new cases of rectal cancer in 2015. The treatment of locally advanced rectal cancers is different from colon cancer in that the standard approach is neoadjuvant chemotherapy, specifically with 5-FU, combined with radiation prior to surgery. This approach was shown to reduce the rate of local recurrence that can cause substantial morbidity and mortality. The achievement of pathologic complete response after neoadjuvant chemo-radiation has also been associated with improved patient outcomes. Both chemotherapy and radiation can cause changes in the intestinal mucosa, in particular changes in intestinal epithelial barrier function and induction of inflammatory responses. In addition chemotherapy and radiation can also affect the composition of the gut microbiome, which, in turn, can affect immune responses that are important for chemotherapy efficacy. Thus, we hypothesize that the composition of the gut microbiome and changes in the activities of the gut microbiome during neoadjuvant therapy are associated with tumor responses. In the proposed work, we will obtain stool samples from locally advanced rectal cancer patients before, during, and after completion of neoadjuvant chemo-radiation and determine the composition and functional activities of the microbiome through 16S bacterial sequencing and mass spectrometry, respectively. The overall goal will be to determine if there are specific bacterial populations or functions that are important for achieving complete pathologic response as assessed by histologic evaluation of tumor specimens surgically removed after completion of neoadjuvant therapy. These studies will lay the foundation for future work to improve tumor responses to chemo-radiation and patient outcomes by manipulation of the gut microbiome.